A non-aqueous secondary battery that includes a non-aqueous electrolyte solution has the advantageous features of a light weight, a high energy, and a long life. Such a non-aqueous secondary battery is widely used as a power source in portable electronic devices, such as laptop computers, mobile phones, smartphones, tablet PCs, digital cameras, video cameras and the like. Further, with the progression toward a society that places less burden on the environment, non-aqueous secondary batteries are drawing attention as a power source for hybrid electric vehicles (hereinafter abbreviated as “HEVs”), plug-in HEVs (hereinafter abbreviated as “PHEVs”), and electric motorcycles, as well as in the field of power storage for household storage systems and the like.
When a non-aqueous secondary battery is mounted in a vehicle, such as an automobile, or in a household storage system, from perspectives such as cycling performance and long-term reliability under a high-temperature environment, the structural materials of the battery need to have excellent chemical and electrochemical stability, strength, corrosion resistance and the like. In addition, the conditions under which the battery is used largely differ from those for portable electronic device power sources. Since such power sources have to operate even in cold climates, a high rate performance and long-term stability under a low-temperature environment are also required as necessary properties.
On the other hand, to meet the needs for higher capacity and higher rate that are expected in the future, not only do materials need to be developed, but it is also necessary to construct the optimum state as a battery that enables each material to exhibit its functions sufficiently. Especially, if an electrode active material layer has a higher volumetric energy density, the diffusion pathway for the lithium ions becomes longer, which means that internal resistance resulting from the insertion and desorption of lithium ions increases. Therefore, to maintain a practical rate performance, a balanced design is necessary.
Generally, it is said that a higher capacity non-aqueous secondary battery can be achieved by improving the performance of the electrode active material. However, in practice, the production of an electrode active material layer having a high volumetric energy density is what is most important. For example, if a large amount of electrode mixture is coated on the electrode current collector, the electrode active material mass per unit volume of the battery becomes relatively greater than that of other battery materials, such as the current collector foil and the separator, that are not related to battery capacity, which means that a higher capacity as a battery can be obtained. Further, if the electrode is pressed at a high pressure, an electrode active material layer having a low porosity can be obtained, which similarly means that a higher capacity as a battery is realized.
When emphasizing the rate characteristic of a non-aqueous secondary battery, such as shortening the charging time, discharging at a large current, or discharging under a low-temperature environment, in contrast to when aiming for a higher capacity, it is necessary to design the electrode active material layer so that the diffusion pathway of the lithium ions is short. Specific examples to do this include reducing the basis weight of the electrode active material layer, increasing the porosity of the electrode active material layer and the like.
By the way, to improve the rate characteristic, it is also effective to select an electrolyte solution having a high ion conductivity. From a practical standpoint, it is desirable to use a non-aqueous electrolyte solution for the electrolyte solution of a lithium-ion secondary battery that operates at ordinary temperature. An example of a common solvent is the combination of a high-permittivity solvent, such as a cyclic carbonate, and a low-viscosity solvent, such as a lower chain carbonate. However, typical high-permittivity solvents have a high melting point, and depending on the type of electrolyte that is used, the rate characteristic, and the low-temperature characteristic can also be deteriorated. One solvent that overcomes this problem that has been proposed is a nitrile-based solvent that has an excellent balance between viscosity and relative permittivity. Among such solvents, acetonitrile is known to be a solvent that has an outstanding performance. However, since nitrile group-containing solvents also suffer from the fatal drawback that they undergo electrochemical reduction and decomposition, several improvement strategies have been reported.
For example, Patent Document 1 reports an electrolyte solution that is less affected by reduction and decomposition, which is obtained by mixing and diluting a cyclic carbonate, such as ethylene carbonate, and a nitrile-based solvent, such as acetonitrile. Further, Patent Document 2 to 4 report a battery that suppresses the reduction and decomposition of a nitrile-based solvent by using a negative electrode that has a higher reduction potential than that of the nitrile-based solvent. In addition, Patent Document 5 reports a non-aqueous electrolyte solution obtained by adding sulfur dioxide and one or more other aprotic polar solvents to a nitrile-based solvent for the purpose of forming a protective film on the negative electrode.